The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The crystalline lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances. This adjustment of the crystalline lens is known as accommodation. The lens capsule is a smooth, transparent membrane that completely surrounds the lens. The lens capsule is elastic and is composed of collagen. The lens is flexible and its curvature is controlled by ciliary muscles through the zonules, which connect the ciliary muscles and the equatorial region of the capsule. At short focal distance the ciliary muscle contracts, the zonules loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power. Changing focus to an object at a greater distance requires the relaxation of the ciliary muscle, which increases the tension on the zonules, flattening the lens and thus increasing the focal distance.
A crystalline lens can be removed and replaced with an artificial lens, generally referred to as an intraocular lens, for a variety of reasons. Some intraocular lenses are used to replace a cataract lens, a clouding that develops in the crystalline lens of the eye, obstructing the passage of light. Intraocular lenses can be characterized as non-accommodating or accommodating. Accommodating intraocular lenses are designed to function similarly to the native crystalline lens and are adapted to change power to provide near and distance vision.
The native crystalline lens is typically removed through a procedure referred to as an extracapsular extraction. The procedure includes making a capsulorhexis, a circular incision made on the anterior side of the capsule, followed by removal of the lens material. The replacement intraocular lens can then be positioned within the capsule through the opening formed by the circular incision.
As is set forth in more detail in U.S. application Ser. No. 12/685,531, filed Jan. 11, 2010, from which this application claims priority, there is patient-to-patient variability in capsular bag size, there are imperfect techniques for measuring capsular sizes, and there are post-implant changes that can occur within the eye or to the accommodating intraocular lens. Accommodating intraocular lenses are desired for which the base state, or base power (which may also be referred to herein as “set-point”), of the accommodating intraocular lens is more predictable after implanting it within an eye, and yet will still accommodate in response to ciliary muscle movement.